Commercial dry cleaning systems currently employ potentially toxic and environmentally harmful halocarbon solvents, such as perchloroethylene. Carbon dioxide has been proposed as an alternative to such systems in U.S. Pat. No. 4,012,194 to Maffei. A problem with carbon dioxide is, however, its lower solvent power relative to ordinary solvents.
PCT Application WO96/27704 (12 Sep. 1996) by Unilever, describes dry cleaning systems using densified carbon dioxide and special surfactant adjuncts. The term "densified carbon dioxide" means "carbon dioxide in a gas form which is placed under pressures exceeding about 700 psi at about 20.degree. C." (pg. 5, lines 1-3). The surfactants employed have a supercritical fluid CO.sub.2 -philic moiety connected to a supercritical fluid CO.sub.2 -phobic moiety (pg 3, lines 30-32). In the method and apparatus described, a vertical rotating drum 5 (FIG. 1) containing soiled fabrics, surfactants, modifier, enzyme, peracid and mixtures thereof is charged with densified CO.sub.2 fluid at a pressure ranging between 700 and 10,000 psi. The CO.sub.2 is then heated to its supercritical range of about 20.degree. C. to about 60.degree. C. by a heat exchanger 4 (pg. 36 line 26 to pg. 37 line 8) and the cleaning cycle initiated. Other densified molecules that have supercritical properties, ranging from methane and ethane through n-heptane to sulfur hexafluoride and nitrous oxide, are noted that may also be employed in the described process, alone or in mixture with CO.sub.2 (pg. 6 lines 25-35).
A problem with the Unilever dry-cleaning technique is that supercritical CO.sub.2 is extremely damaging to some fabrics and buttons used in clothing. In addition, the need for a heater makes the process more energy intensive and expensive, and the need for a container that can hold CO.sub.2 at supercritical temperatures and pressures makes it difficult or impossible to practice the process on conventional dry-cleaning apparatus. Further, because the CO.sub.2 is supercritical, there is no phase boundary in the rotating drum, such as the liquid-vapor boundary found in most traditional dry cleaning process. The presence of the phase boundary in the rotating drum (particularly in horizontal rotating drums) exerts a physical scrubbing and penetrating action on the garments that enhances the cleaning thereof.
U.S. Pat. No. 5,377,705 to Smith et al. describes a precision cleaning system in which a work piece is cleaned with a mixture of CO.sub.2 and a co-solvent. Smith provides an entirely non-aqueous system, stating: "The system is also designed to replace aqueous or semi-aqueous based cleaning processes to eliminate the problems of moisture damage to parts and water disposal" (col. 4 line 68 to col. 5 line 3). Co-solvents that are listed include acetone and ISOPAR.TM. (col. 8, lines 19-24). Use in dry cleaning is neither suggested nor disclosed. Indeed, since some water must be present in dry-cleaning, such use is contrary to this system.
In view of the foregoing, there is a continuing need for effective carbon dioxide-based dry cleaning systems.